Evidence is mounting that some solar cycles are double-peaked.
The ongoing solar maximum may itself be a double -- and the second
peak has arrived.

January
18, 2002: Every 11 years solar activity reaches a fever pitch:
Solar flares erupt near sunspots on a daily basis. Coronal mass
ejections, billion-ton clouds of magnetized gas, fly away from
the Sun and buffet the planets. Even the Sun's awesome magnetic
field -- as large as the solar system itself -- grows unstable
and flips.

It's a turbulent time called Solar Max.

Right: Sunspot counts for the current solar cycle peaked
in mid-2000 and again in late 2001. Image courtesy David Hathaway,
NASA/MSFC.

The most recent (and ongoing) Solar Max crested in mid-2000.
Sunspot counts were higher than they had been in 10 years, and
solar activity was intense. One remarkable eruption on July 14,
2000 -- the so-called "Bastille Day Event" -- sparked
brilliant auroras as far south as Texas, caused electrical brown-outs,
and temporarily disabled some satellites.

After that, sunspot counts slowly declined and the Sun
was relatively quiet for month-long stretches. Solar Max was
subsiding.

But now, as 2002 unfolds, it's back. The Sun is again peppered
with spots, and eruptions are frequent. Says David Hathaway,
a solar physicist at the NASA Marshall Space Flight Center: "The
current solar cycle appears to be double-peaked," and the
second peak has arrived.

Scientists track solar cycles by counting sunspots -- cool
planet-sized areas on the Sun where intense magnetic loops poke
through the star's visible surface. Hathaway is an expert forecaster
of sunspot numbers. "Sunspot counts peaked in 2000 some
months earlier than we expected," he recalls. The subsequent
dip toward solar minimum seemed premature to Hathaway, and indeed
it was. Before long, sunspot counts reversed course and began
to climb toward a second maximum that now appears to be only
a few percent smaller than the first.

Solar
Max eleven years ago was much the same. A first peak arrived
in mid-1989 followed by a smaller maximum in early 1991. In fact,
if the ongoing cycle proves to be a double, it will be the third
such double-peaked cycle in a row.

Sunspots are the most visible sign of those complex magnetic
fields -- but not the only one. Another sign is solar radio emissions,
which come from hot gas trapped in magnetic loops. "The
radio Sun is even
brighter now than it was in 2000," says Hathaway. By
the radio standard, this second peak is larger than the first.

Hathaway notes a widespread misconception that solar activity
varies every 11 years "like a pure sinusoid." In fact,
he says, solar activity is chaotic; there is more than one period.

Earth-directed solar explosions, for instance, tend to happen
every 27 days -- the time it takes for sunspots to rotate once
around the Sun. There is also an occasional 155-day cycle of
solar flares. No one knows what causes it. And the double peaks
of recent solar maxima are separated by approximately 18 months.

The source of all this variability is the
turbulent Sun itself. The outermost third of our star -- the
"convective zone" -- is boiling like hot water on a
stove. California-sized bubbles rise 200,000 km from the base
of the zone to the Sun's surface where they turn over and "pop,"
releasing heat (generated by nuclear reactions in the core) to
space. Below the convective zone lies the "radiative zone"
-- a calmer region where photons, not mass motions, transport
the Sun's energy outward. Says Hathaway: "The Sun's magnetic
field is generated at the boundary between these two layers where
strong electric currents flow."

Right: This artist's concept of the solar interior reveals
the boiling convective zone, the interface layer (where the Sun's
magnetic field is generated), and the relatively calm radiative
zone. [more]

Magnetic fields are produced by electric currents -- that
is, charges in motion. The Sun itself is a conducting fluid.
Our star is so hot that the atoms within it are mostly ionized;
their nuclei are separated from their electrons. As a result,
relative motions between neighboring layers of ionized gas carry
currents and spawn magnetic fields. "The rotational velocity
of the Sun changes suddenly near the convective-radiative boundary,"
says Hathaway. "The velocity shear is what drives the so-called
solar magnetic dynamo."

Below: The false colors in this cutaway diagram of the
Sun represent different gas velocities inside our star. Click
on the image to view a
3MB movie of the 16-month "pulse"
at the base of the convection zone. [more]

Last
year, scientists using a technique called helioseismology, which
can probe conditions within the Sun much like seismic waves reveal
the interior structure of our planet, announced that currents
of gas at the base of the convective zone speed and slacken every
16 months.

"That's about the same as the time between the double
peaks of recent solar maxima," notes Hathaway. Perhaps the
two are connected. "It's hard to be sure," he cautions,
because the detailed inner workings of stellar magnetic dynamos
remain a mystery. "Helioseismology of the Sun, which can
probe beneath its visible surface, is still a young field. We
need more time to understand completely how the internal rhythms
of our star affect the solar cycle."

Whatever the cause, a resurgent Sun is welcome news for many
sky watchers. Solar eruptions can trigger one of the most beautiful
spectacles on our planet: Northern Lights. If the Sun continues
to storm, the skies could be alight, off and on, for many months
to come.

Up & Downs: Solar maxima come every 11 years, but
the ferocity of solar activity isn't the same each time. The
11-year peaks seem to be modulated by an even longer cycle, perhaps
100 years or more in length. During one notable trough called
the "Maunder Minimum" very few sunspots were seen for
6 consecutive solar cycles between 1645 and 1715 -- and Europe
was plunged into the "Little Ice Age." Click to view
sunspot
counts recorded during the past 400 years.

The Most Powerful Solar Flares
ever Recorded
-- (SpaceWeather.com) For reasons scientists don't understand,
the most powerful solar flares erupt most often during the waning
phases of sunspot cycles. Two of the strongest flares on record
happened in early-2001 when the sunspot number was temporarily
declining.

Planetary Dynamo on a Desk -- (SpaceDaily.com) magnetic
dynamos are hard to study because they are hidden deep inside
stars and planets. An innovative device allows one scientists
to study a dynamo on his desktop.